1. Field of the Invention
This invention relates in general to downhole tools used in boreholes and wells, and in particular to tools using fluid-driven acoustical oscillators and circuits to generate pressure fluctuations of large amplitude.
2. Background Information
In U.S. Pat. No. 3,405,770, Drilling Method and Apparatus Employing Pressure Variations in a Drilling Fluid, Oct. 15, 1968, are disclosed improved means for drilling boreholes in the earth by effecting elastic vibrations in the drilling fluid surrounding a rotating drill bit. In the preferred embodiment the fluid pressure at the borehole bottom is cyclically decreased, while simultaneously jet velocity and bit load are cyclically increased through the use of a bistable fluid oscillator, a coupler and resonators which cooperate to generate large fluid pressure fluctuations at the borehole bottom while minimizing acoustical energy transfer upward through the drilling fluid. The output of each of the two legs of the oscillator is fed into a cavity around the bit, after the phase of one output leg is inverted. Acoustical filters in the form of Helmholtz resonators C and D are connected respectively with the fluid passage or axial bore 121 with apertures 119 inside the tool leading to the bit and with the annulus with apertures 131 to minimize pressure losses and enhance efficiency.
Another apparatus and method used to isolate the out-of-phase pressure fluctuations of the output legs of a bistable fluidic oscillator are disclosed in U.S. Pat. No. 3,441,094, Drilling Methods and Apparatus Employing Out-Of-Phase Pressure Variations in a Drilling Fluid, Apr. 29, 1969.
Well stimulation apparatus and methods using the same general approach are disclosed in U.S. Pat. No. 3,520,362, Well Stimulation Method, July 14, 1970, in U.S. Pat. No. 3,842,907, Acoustic Methods for Fracturing Selected Zones in a Well Bore, Oct. 22, 1974, and in U.S. Pat. No. 3,850,135, Acoustical Vibration Generation Control Apparatus, Nov. 26, 1974.
A logging method which utilizes similar apparatus is disclosed in U.S. Pat. No. 3,860,902, Logging Method and system, Jan. 14, 1975, and a system for detecting the position of an acoustic generator in a borehole is disclosed in U.S. Pat. No. 3,876,016, Method and System for Determining the Position of an Acoustic Generator in a Borehole, Apr. 8, 1975.
Field experience and laboratory studies have been used to demonstrate the effectiveness of the above apparatus and methods for generating large pressure fluctuations useful in drilling, well treatment and logging. In the demonstrations a fluidic oscillator was used to alternately direct fluid between an exterior cavity and an interior cavity inside the too, with appropriate acoustical annulus filter means located one-quarter wavelength above, or above and below, the exterior cavity to minimize power dissipation in the annulus. The version of the tool shown in U.S. Pat. No. 3,520,362 is being used with successful results for well stimulation in a cased hole.